1. Field of the Invention
The present invention generally relates to formation of memristor material, and more particularly to formation of both memristor material and electrode structure with memristance by using ion implantation.
2. Description of the Prior Art
Memristors (“memory resistors”) are a class of passive two-terminal circuit elements that maintain a functional relationship between the time integrals of current and resistance. Its function, the memristance, is essentially similar to the variable resistance, when a memristor's memristance is a function of charges inside the memristor. However, there is no generic memristor. Indeed, different types of such devices implement some particular functions.
The HP Laboratory announced a type of memristor on Apr. 30, 2008. The memristor presented by HP is composed of a thin titanium dioxide film between two electrodes. Herein, as example, the film is as thin as three nanometers, and the electrodes are a pair of platinum layers. The film includes two layers, wherein one is a non-depleted layer and the other is a depleted layer which has a slight depletion of oxygen. The oxygen vacancies where oxygen atoms would normally act as charge carriers, meaning that the depleted layer has a much lower resistance than the non-depleted layer. When the electrodes apply a current through the thin titanium dioxide film, the oxygen vacancies will be driven so as to let the distribution of the oxygen vacancies change. In other words, the resistance of the film as a whole is dependent on how much charge has been passed through the film in a particular direction, which is reversible by changing the direction of current. Clearly, when all oxygen vacancies drift to an interface between the film and one electrode, the resistance of the film is maximum because there is no charge carrier inside the film.
Note that the key of the variation of the resistance of the memristor is how the distribution of the charge carriers (such as the oxygen vacancies) is changed by the net charges passing through the memristor. Hence, the thin titanium dioxide film is variable. In other words, the film can have two layers or only one layer having oxygen vacancies. Moreover, owing to the similar resistance transition characteristics of the transition metal oxides, the titanium dioxide film can be even replaced by a transition metal oxide (TMO). For example, zinc oxide (ZnO), titanium dioxide (TiO2), niobium oxide (Nb2O5), zirconium dioxide (ZrO2), or nickel oxide (NiO)
Conventional manufacturing methods of the transition metal oxide film include sputtering, evaporation, physical vapor deposition (PVD) process, chemical vapor deposition (CVD) and Sol-Gel. In general, the wafer is placed in a chamber and the components of the transition metal oxide film are directly formed on the surface of the wafer. However, the transition metal oxide film formed by evaporation usually is more porous and has lower density, despite having more oxygen vacancies. The transition metal oxide film formed by sputtering or PVD usually has higher density, but it is easier to be polluted by the used plasma and has less oxygen vacancies. The transition metal oxide film formed by CVD is easier to be polluted by the used precursors. The transition metal oxide film formed by the Sol-Gel usually has lower density and higher porosity. Moreover, all these conventional methods can not precisely and flexibly change the chemical composition of the transition metal oxide film. Besides, all these conventional methods are fine tuned for forming a transition metal oxide film with at least a hundred angstroms thickness, but they are not proper to form a transition metal oxide film with several nanometers thickness.
Clearly, all conventional manufacturing methods are not perfect enough to form a high-quality transition metal oxide film, especially when the required thickness of the transition metal oxide film is only several nanometers. Besides, the amount and the distribution of oxygen vacancies inside the film are hard to be precisely and flexibly adjusted, because these oxygen vacancies are formed with growth the transition metal oxide film. Therefore, the applications of the memristor with transition metal oxide film may be degraded and restricted.
Accordingly, it is desired to develop a new approach to form memristor material with transition metal oxide film, wherein both the amount and the distribution of the oxygen vacancies inside the transition metal oxide film can be effectively and flexibly adjusted.